1. Field of the Invention
This invention relates to an apparatus and a method for manufacturing a microwave circuit module, and more particularly to a method of assembling and checking microwave circuit modules which are used for microwave communication systems and the like.
An MMIC (microwave monolithic integrated circuit) is now in the stage of practical utilization. An ultra-high frequency integrated circuit is greatly different from the general digital IC in that it cannot be realized only with lumped parameter circuits but requires distributed constant circuits.
However, the above integrated circuit is not suited for integrating therein distributed constant circuits in a large scale. The distributed constant circuit has a size determined by the wavelength, and it usually requires a great space for installation or mounting.
The increased size in the MMIC is disadvantageous from the points that gallium arsenide (GaAs) is often utilized, that thin wafers are preferred in view of characteristic requirements, and that the price becomes high. In these respects, the MMIC is not suited for the large-scale integration.
As means for solving the above problems and economically realizing large-scale microwave circuits including distributed constant circuit elements, the inventor has proposed "Composite Microwave Circuit Module" (Japanese Patent Application No. Hei 4(1992)--291031 which was filed in Japan on Oct. 29, 1992 and which has not been published yet).
FIG. 10 shows the composite microwave circuit module disclosed in the patent application referred to above. As shown therein, a dielectric substrate which is formed as a lamination of multiple layers is provided with ground surfaces formed of conductive layers comprised of an upper and a lower layer 26 and 23, and signal circuits 24 and 25 formed of one or more conductive layers for signal transmission and disposed in an intermediate layer portion of the substrates.
Further, in this composite microwave circuit module, a shield surrounding the signal circuits 24 and 25 is formed by thorough-holes 32 and 84 which are formed around the signal circuits 24 and 25 for short-circuiting the two ground surfaces. These circuits include active elements 28, such as MMICs, ICs, transistors, diodes, etc., and passive elements, which are formed by patterning of strip lines, microstrip lines, coplanar lines, etc. these elements being electrically connected to one another to obtain an integrated structure.
Referring to FIG. 10, designated at 20 is a base plate, at 21 a layered capacitor, at 22 a conductive cap, at 27 a wiring or interconnect layer, at 29 a signal line pattern, at 30 a power source pattern, at 31 a bonding wire, at 33 a chip installation land, at 35 a conductor pattern, and at 36 a land.
In the above composite microwave circuit module, the active elements and the divided connection substrate for interconnecting these elements are realized by use of interconnect patterns within the integrated multi-layer substrate, whereby the characteristic deterioration caused by connection is precluded and the number of process steps in the fabrication is reduced.
Further, in the above composite microwave circuit module, the interference due to coupling of the microwave circuits that results from the accommodation of the circuits in a common case and the resonance that results from the case as a cavity are avoided by the provision of a shield structure or a cavity structure accommodating active elements.
Also, in the above composite microwave circuit module, integration inclusive of peripheral circuits makes a reduction in a large number of mounting stage layers which have heretofore been necessary, thus attaining the reduction of the size and cost. This composite microwave circuit module is complementary to the MMIC techniques which are otherwise not suited for the assembling of passive elements for large-scale integration including passive elements and lines.
In realizing the above cavity structure, a dielectric portion above the intermediate layer portion used for signal transmission is removed thereby forming a cavity as a place to install such semiconductor elements as MMICs. The semiconductor elements are installed thereat and the ground surface removed at the top is covered with a conductive cap.
Further, the above seal structure of the composite microwave circuit module realizes an air-tight seal with the conductive cap and provides a protective structure with the conductive cap, whereby the semiconductor elements installed are protected from contamination by external fields.
The above composite microwave circuit module has the following other features.
Firstly, the composite microwave circuit module is with a structure, in which a conductor land (an element installation surface) is provided in the intermediate layer portion and connected to a lower layer ground surface via a plurality of via-holes (for the reduction of RF impedance), and the via-holes are filled with a metal to reduce thermal resistance with respect to the lower layer and to obtain effective heat conduction.
Secondly, the composite microwave circuit module is with a structure, in which the via-holes are provided right underneath a local heat generation spot such as a transistor region in a semiconductor chip for thermal resistance reduction.
Thirdly, the composite microwave circuit module is with a structure, in which a plurality of conductor layer lands are provided as alternate capacitor and ground lands between an element-installation surface of the intermediate layer portion and a lower ground layer, with the capacitor lands connected together and the ground surface and the ground lands connected together via via-holes, thus forming a laminate capacitor which functions as a bypass capacitor for an installed element (mounted chip), while dissipating heat laterally through the laminated conductor layer lands to reduce the thermal resistance.
In the fourth place, the composite microwave circuit module is with a structure, in which the intermediate layer portion is provided with a very thin dielectric layer which forms, together with the conductor patterns on both sides of such dielectric layer, capacitive coupling, thus forming a transmission line to cut direct current (DC) component.
In the fifth place, the composite microwave circuit is with a structure, in which a dielectric layer is formed on an upper ground layer with conductor patterns formed thereon as wiring patterns to obtain power supply wirings.
In the sixth place, the composite microwave circuit module is with a structure, in which, at a portion connected to inner layers, a land is provided to form together with the immediate vicinity ground layer a capacitor as a bypass capacitor to prevent RF leakage from the inner layers.
The above composite microwave circuit module is manufactured with a technique which permits realizing a large-scale microwave circuit in a compact form. However, it includes a plurality of active elements and these active elements are subject to characteristic fluctuations in manufacture. Therefore, an improvement is called for in this area in order to enhance the yield of the overall manufacturing of the composite microwave modules.
As an example, where it is desired to obtain an overall yield of 90% or above of the circuit module which includes a total of five active elements, such as amplifier, oscillator, mixer, etc., and if the yield is simply distributed to the five active elements, each element should be given a yield of 98% or above. If the yield is also distributed to the assembling process, the yield for each element is even higher.
The characteristic fluctuations are inevitable in the active elements, and the reason therefor is that they are ultra-high frequency integrated circuits using gallium arsenide and are also analog circuits.
Therefore, it is unfeasible to require an yield of 98% on standards inclusive of those of the characteristic fluctuations of the active elements. Such a requirement would result in a low proportion of active elements as units falling within the rated fluctuation standards. If it is desired to obtain a composite microwave circuit module with an economically high yield, therefore, a special measure has to be considered.
A generally conceivable method to this end is to check electrical characteristics of the composite microwave circuit module after the installation of the active elements and adjust microstrip lines around the active elements according to the check results, thereby obtaining a desired performance.
This method, however, has a problem in that it is difficult to carry out the adjustment after completion of the assemblage of the module constituting a large-scale circuit. Another problem is that the active elements are apt to be contaminated so that, since they cannot be given any seal before completion of the adjustments, the reliability thereof is lowered.
Further, the adjustment of microstrip lines around the active elements requires a high skill and a long time, thus leading to a high price.